1. Field of the Invention
The present invention relates to a stepper motor assembly. More particularly, the invention relates to a control system and a method for detecting a stall condition in a stepper motor assembly.
2. Description of the Prior Art
A stepper motor is a type of electrical motor that can be incrementally stepped between various rotational positions. Stepper motors generally include a rotatable permanent magnet, a first inductive coil and a second inductive coil. The first and second inductive coils are in electrical communication with a control system, which selectively energizes and de-energizes the coils. When one of the coils is energized, it produces an electro-magnetic field to rotate the permanent magnet. The control system can either direct the stepper motor to take a “full-step” by energizing one coil while the other coil remains de-energized or a “micro-step” by substantially continuously driving both of the coils with out-of-phase voltages.
The control system for the stepper motor is generally an open loop system. In other words, the control system sends voltages to the first and second inductive coils to move the stepper motor, but the stepper motor does not communicate its absolute position back to the control system. To ensure the accuracy of the stepper motor, it is often necessary for the control system to perform a referencing operation on the stepper motor before each use of the stepper motor. Many stepper motor assemblies include a stop, either internal or external of the stepper motor, for stalling the stepper motor in a known position during the referencing operation. However, the control system still must be able to determine when the stepper motor is in a stall condition in order to ensure that the stepper motor is in the known position.
One way for the control system to determine that the stepper motor is in the stall condition is to measure the back electro-motive force (EMF) of the first and second inductive coils. In other words, when the control system is energizing one of the inductive coils and the other inductive coil is de-energized, the rotation of the permanent magnet will induce a voltage in the de-energized coil. This voltage is the back EMF, and it can be measured by the control system. A large back EMF reading means that the permanent magnet is spinning, and thus, the stepper motor is not in a stall condition. In contrast, a small back EMF reading means that the permanent magnet is stationary, and thus, the stepper motor is in a stall condition.
One process for determining that the stepper motor is in a stall condition is to rotate the stepper motor in one direction until it reaches a stop while monitoring the back EMF with the control system. Once the back EMF falls below a predetermined threshold, then the control system rotates the stepper motor back in the other direction while again monitoring the back EMF until it reaches a second stop, at which point, the back EMF again falls below a predetermined threshold. This referencing operation is generally known as a “welcome sweep”.
The welcome sweep operation is undesirable for many applications, particularly when the stepper motor is coupled to an instrument pointer, e.g., a speedometer, a tachometer, etc. This is because the instrument pointer must sweep through a wide rotation range before the instrument pointer can accurately indicate the desired information, and this process takes an undesirable amount of time to execute. Even further, the welcome sweep operation is undesirable in the instrument panel of an automobile, which may include many instrument pointers (e.g. a speedometer, a tachometer, a fuel gauge, an oil pressure, a battery voltage, a coolant temperature, etc.), each driven by its own stepper motor.
There remains a significant and continuing need for a stepper motor assembly which can be homed to a predetermined reference position with little to no rotational movement.